1. Field of the Invention
This invention relates to a lead screw for moving a head, such as a printing head in a printer or a magnetic head in a magnetic disc apparatus.
2. Description of the Prior Art
There is known a lead screw for moving a printing or magnetic head. It has a helical groove of which the sidewalls are perpendicular to the outer peripheral surface of the screw, and which is, therefore, rectangular in cross section. A typical lead screw is shown fragmentarily at 1 in FIGS. 1 and 2. It has a helical groove 2 which is rectangular in cross section, as shown in FIG. 2. The outer edges 3 of the groove 2, therefore, make partly invisible the inner or bottom edges 4 and the bottom of the groove 2 as indicated at 5 in FIG. 1, when the screw 1 is, for example, looked at in front elevation. The invisible portions 5 are usually called undercuts, and have heretofore made it impossible to mold the screw 1 from plastics. There has been no alternative but to make the groove 2 by cutting. The cutting of a helical groove, however, requires a high level of technique and a lot of time, and results, therefore, in an expensive lead screw.